TRIASSIC. 531 



Darton ^**'' has traced the Chugwater throughout central Wyoming and, 

 describing it under the heading 'Termian-Triassic," gives its general relations as 

 follows : 



The Chugwater formation ranges in thickness from 900 to 1,200 feet and consists of sandy 

 shales or soft, massive sandstones, nearly all of bright-red color. Gypsum deposits occur in 

 most places. There are extensive exposures along the lower northeastern slope of the Wind 

 River Mountains in the Owl Creek, Bridger, and Rattlesnake uplifts, in the flexures south of 

 Casper and Douglas, above Alcova, in the Shirley and Freezeout hiUs, in the anticlines north 

 and east of Medicine Bow, and along the east side and across the south end of Laramie Basin. 

 It appears also at intervals along the east slope of Laramie Mountains. In the greater part of 

 the south-central Wyoming region the Chugwater formation lies on Tensleep sandstone, but 

 in the southern end of the Laramie Basin it lies on red beds of the Casper formation, from 

 which in places it is not easily separated. 



Detailed descriptions of the formation for all the ranges mentioned are given 

 in the paper cited. 



In the red beds thus placed by Darton in the Chugwater formation, in the 

 ranges between Rawlins and Lander, Wyo., Williston ""'' has traced a series of beds 

 near the top of the formation carrying Triassic vertebrates. He says: 



The University of Chicago paleontological expedition to Wyoming the past summer was 

 fortunate in securing a valuable collection of stegocephalian and reptilian remains from the 

 Trias, a part of which is described in the present paper. 



The beds whence the fossils were obtained, from 40 to 80 feet in tliickness, are about 200 

 feet below the top of the red beds and about 600 feet above their base. Their description will 

 be given in a later paper by Mr. N. H. Brown, their discoverer, and the writer. Meanwhile the 

 horizon may be distinguished by the name Popo Agie beds, as suggested by Mr. Brown, from 

 the Popo Agie River, along whose branches they are most characteristically shown. 



The forms described comprise Dolichobrachium gracile, Euhrachiosaurus hroumi, 

 Brachybrachium brevipes, and Paleorhinus bransoni. 



Williston ^*^^ also found Triassic vertebrates in the red beds (Chugwater) of 

 Red Mountain south of Laramie. He says : 



We found there numerous fragments of bones scattered along a thin stratum near the top 

 of the red beds. The marine Jurassic is here wanting, as at Canon City, the sandstone of the 

 Morrison or Atlantosaurus beds overlying the red beds without marked unconformity. The 

 lower members of these beds consist of a grayish or yellowish sandstone and are unfossiliferous, 

 the first vertebrate fossUs occurring 75 feet or more above the red beds horizon. [This stratum 

 yielded] very characteristic labyrinthodont plates and vertebrse, proving conclusively the 

 Triassic age of the deposits. 



The difl^erences of these fossils from those obtained in the Lander region [Popo Agie beds of 

 Williston] from a horizon fully 250 feet below the top of the beds, are such that their contempo- 

 raneity of deposition is very improbable. * * * j ^^^^ much inclined to believe that the 

 Popo Agie beds, which may be contemporaneous with those yielding vertebrate fossils in Utah, 

 Arizona, New Mexico, and Texas, are of early Keuper age, while the Connecticut Valley, the 

 Red Mountain, and Hallopus beds of southern Colorado are later in time. 



Darton in a later note ^*^ states that the red beds of Red IMountain grade into 

 upper Carboniferous sandstones and limestones, which along the Rocky IVIountain 

 front become the Lower Wyoming division of Eldridge and the Fountain formation 

 of Gilbert and Cross. Darton cites Knight's discovery of upper Carboniferous 

 fossils in the red beds of the Laramie Basin and concludes that the evidence is con- 

 tradictory. 



